The present disclosure relates generally to providing an apparatus, a method, and an electrolytic solution for electropolishing products made from metals. In particular, the present disclosure relates generally to electropolishing metallic medical devices (e.g. stents) made of high-strength medical alloys, such as stainless steel, titanium, nickel-titanium, tungsten, tantalum, niobium, cobalt-chromium, cobalt-chromium-tungsten, etc. While the apparatus, method, and electrolytic solution are described herein as being applicable mainly to medical stents, in particular intravascular stents, the disclosure is not limited to such medical products. For example, the methods may be applied to electropolish metallic automotive or aerospace components.
Stents are generally tube-shaped devices placed within a blood vessel or other body lumen to maintain the patency of the lumen and, in some cases, to reduce the development of restenosis. The stents may be formed in a variety of configurations which are typically expandable since they are delivered in a compressed form to a desired site. Such a configuration may be a helically wound wire, wire mesh, weaved wire, serpentine stent, chain of rings, other configuration, or combinations thereof. The walls of stents are typically perforated in a framework design of wire-like connected elements or struts, or in a weave design of cross-threaded wire.
Some stents are made of more than one material. For example, a stent may include a sandwich of metals having outer layers of a biocompatible material, such as stainless steel or cobalt-chromium, with an inner layer providing the radiopacity to the stent for tracking by imaging devices during placement. A stent made of such material may be, for example, a thin layer of titanium between layers of stainless steel or cobalt-chromium. In forming such stents from metal, a roughened outer surface of the stent may result from the manufacturing process. It may be desirable for the surface of the stent to be smooth so that it may be inserted and traversed with reduced friction through the blood vessels or other body lumens toward the site of implantation. A rough outer surface may not only increase frictional obstruction, but may also damage the lining of the vessel wall during insertion. Furthermore, smooth surfaces may reduce thrombus formation and/or corrosion.
Since processing to form metallic stents often results in a product initially having burrs, sharp ends, debris, slag material from melting the metal during processing, other features, or combinations thereof, as a first order treatment of the product, the surface may be descaled in preparation for, for example, further surface treatment such as electropolishing.
An apparatus, a method, and an electrolytic solution is provided for electropolishing stents after they have been descaled, for example, as disclosed in U.S. patent application Ser. No. 11/370,642 filed on 7 Mar. 2006, the disclosure of which is hereby incorporated, in its entirety, by this reference.
Descaling may include, for example, dipping the stent into a strongly acidic solution and/or ultrasonically cleaning the stent.
Electropolishing is an electrochemical process by which some of the surface metal may be electrolytically dissolved. In general, the metal stent serves as an anode and is connected to a power supply while immersed in an electrolytic solution having a metal cathode connected to the negative terminal of the power supply. Current flows from the stent, as the anode, causing it to become polarized. The rate at which the metal ions on the stent are dissolved may be controlled by the applied current and/or voltage. The positioning of the cathode relative to the stent may provide an even distribution of current to the stent. According to the theory of electropolishing, the current density is typically highest at high points protruding from a surface and is typically lowest at the surface low points. Thus, the higher current density at the raised points may cause the metal to dissolve faster at these points, which may level the surface. Electropolishing therefore may smooth the surface, even to the point where it is shiny and reflective.
The present disclosure provides an apparatus, a method, and an electrolytic solution for electropolishing a plurality of metallic devices (e.g., stents) substantially simultaneously to consistently produce smooth surfaces.